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1. Field of the Invention
This invention pertains generally to the construction of pencil ignition coils used in the automotive industry, and more particularly to methods of preventing case erosion in pencil coils having a secondary wound external to the primary and the shields located external to the case.
2. Description of the Background Art
Automotive pencil ignition coils known in the art are generally used to transform voltage from the 12-volt battery to a substantially higher voltage needed for ignition. Such ignition coils typically have a core with primary and secondary windings. A pencil ignition coil that has a secondary winding external to its primary winding and a shield located external to a case will yield a higher energy capability than one where the primary is wound external to the secondary. The higher energy capability is obtained because the primary winding may be wound onto a core, which eliminates the need for a spool and allows for a larger core and greater energy density.
A known pencil ignition coil configuration is generally shown in FIG. 1 and FIG. 2. FIG. 1 is a schematic diagram showing the basic configuration of an ignition coil 10, having a primary winding 12 around core 14 which is internal to a secondary winding 16. These elements are housed in a case 18 covered by a shield 20, wherein an air gap 22 is located between the case 18 and the shield 20. The air gap 22 is not drawn to scale.
Unfortunately, a pencil ignition coil having a configuration as shown in FIG. 1 may suffer damage to the case 18 because of the high electrical field that occurs between the case and the shield. As shown in FIG. 2, partial discharge arises from ionization of air 24 in the gap 22 located between the case 18 and the shield 20, producing ozone (O3) 26. This ionization of air 24 into ozone 26 is called the phenomenon of xe2x80x9cpartial dischargexe2x80x9d and occurs whenever an air gap 22 wider than approximately five microns experiences the presence of a high electric field as is typically created by a pencil ignition coil 10 having an internal primary winding 12, an external secondary winding 16, and a shield 20 located external to the case 18. Partial discharge causes case-erosion, resulting in an eroded layer 28, which leads to eventual failure of the entire pencil ignition coil 10. Therefore, there is a need for a method of inhibiting case erosion in pencil ignition coils that has an internal primary winding. The present invention satisfies that need, as well as others, and overcomes the deficiencies found in conventional coils.
The present invention comprises a protected erosion-resistant pencil ignition coil assembly and a method for fabricating the erosion-resistant pencil ignition coil assembly. The present invention also comprises several means for resisting damage due to the partial discharge phenomenon: (1) fabricating a pencil ignition coil having a coating for resisting erosion of the case by ozone; (2) fabricating a pencil ignition coil having a coating for eliminating partial discharge under a shield; and (3) fabricating a pencil coil having a polyimide tape covering for resisting partial discharge under a shield. A polyimide tube may also be used, assembled over the case and under the shield.
By way of example, and not of limitation, according to one aspect of the invention, the case is coated with a material that is resistant to the ozone that is produced when the air ionizes upon partial discharge. Suitable materials include, but are not limited to, titanium dioxide or silicon dioxide in a base such as paint, E-coat, polyester, or any other material that acts as a carrier and adheres to the case when dried or cured.
According to another aspect of the invention, the case is either electroplated or coated with a substrate material such as paint, polyester, epoxy or the like that is highly filled with a conductive material. Suitable conductive materials include, but are not limited to, carbon black, aluminum, or iron. This coating would be in electrical contact with the shield that is also grounded and in intimate contact with the case. With the coating and the shield at the same potential, no field would exist between them and, therefore, the air could not ionize to produce ozone. This would also reduce electrical noise since the partial discharges are eliminated.
According to a still further aspect of the invention, one or more layers of Kapton CR(trademark) tape, or other corona resistant polyimide, are attached to the case between the case and the shield. For example, a one half millimeter to three millimeter thick base polyimide material with a one half millimeter to two millimeter thick glue coating would be suitable. The tape would be pressed onto the case so that the glue displaces the air between the tape and case. The air between the tape and the shield may still ionize, but the tape would resist damage to the case from the partial discharge that would result from the ionization.
An object of the invention is to inhibit ozone caused case erosion in pencil coils having an internal primary winding.
Another object of the invention is to inhibit case damage from partial discharge in pencil coils having an internal primary winding.
Another object of the invention is to eliminate partial discharge in pencil coils having an internal primary winding. Another object of the invention is to provide for increased commercial value by allowing the use of an efficient internal primary winding that increases energy density.
Another object of the invention to eliminate the need for a spool in pencil coils, thereby lowering the manufacturing costs.
Another object of the invention is to provide for ease of processing by coating or molding.
Another object of the invention is to reduce electrical noise.
Another object of the invention is to improve durability even through thermal cycling.
Another object of the invention is to lower case manufacturing costs by allowing the use of lower cost plastics.
Further objects and advantages of the invention will be brought out in the following portions of the specification, wherein the detailed description is for the purpose of fully disclosing preferred embodiments of the invention without placing limitations thereon.